Vehicle differential including pump with variable-engagement clutch

ABSTRACT

A vehicle differential assembly is provided that includes a differential adapted to allow differing rotational speed between a pair of outputs. The differential includes a gear assembly connected to the outputs and one or more hydraulically-actuated clutches for selectively and variably coupling the outputs. A hydraulic pump is adapted to generate hydraulic fluid pressure for engagement of the hydraulically-actuated clutches. A variable-engagement clutch is operatively connected to the input and the hydraulic pump such that the input selectively drives the hydraulic pump during engagement of the clutch to provide hydraulic fluid pressure to the hydraulically-actuated clutches. A valve operatively connected to the hydraulic pump and the hydraulically-actuated clutches to selectively and variably provides fluid pressure from the hydraulic pump to the hydraulically-actuated clutches. A method of controlling vehicle stability is also provided.

RELATED APPLICATION DATA

This application is a divisional of U.S. Ser. No. 11/412,764, filed Apr.27, 2006, which is a continuation-in-part of U.S. Ser. No. 11/223,568,filed Sep. 9, 2005, now U.S. Pat. No. 7,361,114.

FIELD OF THE INVENTION

The present invention relates to a vehicle differential and to a vehicledifferential including a hydraulic pump with a variable-engagementclutch.

DESCRIPTION OF THE RELATED ART

Differentials are provided on vehicles to permit an outer drive wheel torotate faster than an inner drive wheel during cornering as both drivewheels continue to receive power from the engine. While differentialsare useful in cornering, they can allow vehicles to lose traction, forexample, in snow or mud or other slick mediums. If either of the drivewheels loses traction, it will spin at a high rate of speed and theother wheel may not spin at all. To overcome this situation,limited-slip differentials were developed to shift power from the drivewheel that has lost traction and is spinning to the drive wheel that isnot spinning.

Recently, an electronically-controlled, limited-slip differential hasbeen introduced that includes a hydraulically-actuated clutch to limitdifferential rotation between output shafts of the differential. Thehydraulically-actuated clutch is powered by a pump connected to avehicle drive shaft. Most of the time, the vehicle has adequate tractionnegating the need to actuate the hydraulic clutch. However, provided thedrive shaft is rotating, the pump is still operating and pumping fluid.In this arrangement, the differential requires one or more valves todistribute pressurized fluid to the hydraulically-actuated clutch whenneeded. The parasitic energy losses generated by the continuallyoperating pump can negatively impact vehicle fuel economy and shortenthe useful life of the hydraulic fluid. For at least these reasons, animproved differential is desired.

SUMMARY OF THE INVENTION

A vehicle differential assembly is provided that includes a differentialdriven by an input and adapted to allow differing rotational speedbetween a pair of outputs. The differential includes a gear assemblyconnected to the outputs and one or more hydraulically-actuated clutchesfor selectively and variably coupling the outputs. A hydraulic pump isadapted to generate hydraulic fluid pressure for engagement of the oneor more hydraulically-actuated clutches. A variable-engagement clutch isoperatively connected to the input and the hydraulic pump such that theinput can selectively drive the hydraulic pump during engagement of theclutch to provide hydraulic fluid pressure to the one or morehydraulically-actuated clutches. A valve is operatively connected to thehydraulic pump and the one or more hydraulically-actuated clutches toselectively or variably provide fluid pressure from the hydraulic pumpto the one or more hydraulically-actuated clutches. Other aspects of theinvention will be apparent to those skilled in the art after review ofthe drawings and detailed description provided below. A method ofcontrolling the stability of a vehicle is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a vehicle powertrain systemincluding a differential assembly and torque coupling according toembodiments of the present invention;

FIG. 2 is a schematic illustration of a differential assembly accordingto an embodiment of the present invention;

FIG. 3 is a schematic illustration of the differential assembly of FIG.2 shown during engagement of a variable-engagement clutch and ahydraulically-actuated clutch;

FIG. 4 is an enlarged view cross-sectional view of thevariable-engagement clutch shown in FIGS. 2 and 3;

FIGS. 5A and 5B are schematic illustrations of a medium duringdisengagement and engagement, respectively, of the variable-engagementclutch shown in FIGS. 2-4;

FIG. 6 is a cross-sectional view of a variable-engagement clutchaccording to another embodiment of the present invention;

FIG. 7 is a schematic illustration of a torque coupling according to anembodiment of the present invention;

FIGS. 8A-8D are schematic illustrations of a differential assemblyaccording to an embodiment of the invention; and

FIG. 9 generally illustrates a logic diagram for vectoring torque tocontrol the stability of a vehicle according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

Referring now to the drawings, several embodiments of the presentinvention are shown. The drawings are not necessarily to scale andcertain features may be simplified or exaggerated to better illustrateand explain the present invention. Further, the embodiments set forthherein are not intended to be exhaustive or otherwise limit or restrictthe invention to the precise configurations shown in the drawings anddisclosed in the following detailed description.

Referring to FIG. 1, an exemplary automotive vehicle 10, such as apassenger car, sport utility vehicle or light truck, is shown thatincludes first and second vehicle axles 12 and 14, respectively, a primemover 15, such as an internal combustion engine, and a powertransmission mechanism 18. In the illustrated embodiment, second axle 14serves as the primary vehicle-propelling drive axle to which primarydrive wheels 16 are operatively connected. In contrast, first axle 12serves as a secondary axle to which a steering wheel may be connected.Optionally, first axle 12 may also function as a vehicle-propellingdrive axle adapted to receive torque from a transfer case 19 (shown inphantom in FIG. 1) that apportions torque between primary and secondarydrive axles 14, 12. Torque may be transmitted to axles 12 and 14 throughone or more prop- or drive-shafts 20, an optional torque coupling 21according to an embodiment of the present invention, and a differentialassembly 22 according to an embodiment of the present invention. Vehicle10 shown in FIG. 1 is provided by way of example only and may includeother powertrain arrangements, such as, for example, a primary frontdrive arrangement in which first axle 12 serves as the primary driveaxle.

Referring to FIGS. 2 and 3, an embodiment of differential assembly 22 isshown. In the illustrated embodiment, differential assembly 22 is ahydraulically-assisted, electronically-controlled, limited-slipdifferential that is capable of providing variable torque distributionbetween a pair of outputs 28 and 30, which, if desired, can range up tofull axle lock. Differential 22 may be used as a stand-alone product or,instead, if desired, may be integrated with another vehicle system, suchas a vehicle's antilock brake system (ABS) or stability control package,to provide enhanced vehicle dynamics.

As shown in FIG. 2, differential assembly 22 includes a differential 24that is driven by an input 26 and adapted to allow differing rotationalspeed between outputs 28, 30. A gear assembly 32, which may include apair of side gears 34 and 36, is connected to a corresponding one ofoutputs 28, 30. A ring gear 38 may include a pair of pinion gears 40that, for example, may mesh with side gears 34, 36. Input 26 includes arotatable pinion shaft having a pinion gear 42 the meshes with ring gear38.

A hydraulically-actuated clutch 44 selectively and variably couplesoutputs 28, 30 when torque transfer therebetween is desired. In theillustrated embodiment, hydraulically-actuated clutch 44, which is shownschematically for illustration, includes a multi-disk clutch pack 46 anda clutch pack-compressing actuator 48 (e.g., a piston) that is movablein response to application of hydraulic fluid pressure. At least onefirst friction disk 50 is connected for rotation with output 30 and atleast one second friction disk 52 is connected for rotation with output28. The embodiment of hydraulically-actuated clutch 44 generically shownin FIGS. 2 and 3 is provided by way of reference only and it will beappreciated that other fluid power assisted clutch configurations thatselectively and variably couple outputs 28, 30 may be employed indifferential assembly without departing from the scope of the presentinvention. For example, hydraulically-actuated clutch 44 may compriseany type of axially actuated clutch, such as a single disk clutch, amulti-disk clutch or a cone clutch. A square jaw clutch or a spiral jawclutch may also be used; however, these clutches do not necessarilyproportionally transmit torque.

Differential assembly 22 also includes a hydraulic pump 54 adapted togenerate hydraulic fluid pressure for engagement ofhydraulically-actuated clutch 44. In an embodiment, hydraulic pump 54 isa gerotor pump driven by a variable-engagement clutch 56 operativelyconnected to input 26 such that input 26 selectively and variably driveshydraulic pump 54 during engagement of variable-engagement clutch 56. Tominimize parasitic losses associated with operation of hydraulic pump 54when pressurized fluid is not needed, variable-engagement clutch 56 maybe operated only when fluid power is required by hydraulically-actuatedclutch 44 and the level of engagement is tailored to the degree ofengagement desired in hydraulically-actuated clutch 44. In this manner,the valves and other apparatus required to distribute fluid power invarious conventional electronically-controlled, limited-slipdifferentials may be eliminated. A housing (not shown) may surrounddifferential 24, hydraulic pump 54, and variable-engagement clutch 56,and may include a sump 58 from which hydraulic pump 54 draws hydraulicfluid for pressurization and transfer to hydraulically-actuated clutch44.

In an embodiment, variable-engagement clutch 56 may be a magneticparticle clutch that transmits torque between input 26 and hydraulicpump 54 in proportion to the electric current supplied to it. Whilevariable-engagement clutch 56 is generally described herein as amagnetic particle clutch, other types of variable-engagement clutches,such as clutches that employ electro-restrictive media to transmittorque between two relatively rotating members, may also be used indifferential assembly 22 without departing from the scope of the presentinvention.

In the illustrated configuration, variable-engagement clutch 56 issupported on input 26 by bearings 60 that may be positioned within agenerally cylindrical support 62 that can be attached to the housingsurrounding differential 24, hydraulic pump 54 and variable-engagementclutch 56. As generally shown in FIG. 4, a rotatable and generallycylindrical input member 64 may be operatively connected to input 26, arotatable and generally cylindrical output member 66 is operativelyconnected to hydraulic pump 54. A medium 68 (such as a rheologicalmedium; see, e.g., FIGS. 5A and 5B) is disposed between input and outputmembers 64, 66. In the illustrated embodiment, input and output members64, 66 are splined to input 26 and hydraulic pump 54, respectively. Anynumber of other bearings, such as bearings 69, may be used to facilitaterotation of input and output members 64, 66 relative to input 26 and/orthe differential assembly housing.

Input and output members 64, 66 generally exhibit magnetic properties,but may include at least one generally non-magnetic feature 70. As willbe described in further detail below, magnetic flux will follow a pathof least resistance (i.e., a path of highest magnetic permeance).Non-magnetic features 70 make the path through medium 68 and outputmember 66 an easier magnetic path (with higher magnetic permeance) thanthe short circuit through input member 64.

Referring to an embodiment shown in FIG. 4, non-magnetic feature 70 maycomprise a groove having a generally trapezoidal cross-section, but isnot necessarily limited thereto. Alternatively, non-magnetic feature 70may comprise, for example, a non-magnetic ring or slot disposedsubstantially or completely through the corresponding input or outputmember 64, 66. Moreover, non-magnetic feature 70 may be disposed on asurface of input or output member 64 or 66, or, may be disposedsubstantially or completely through input and output members 64, 66. Thenumber of magnetic features 70 included in input and output members 64,66 may depend on, for example, the torque transferring requirements ofclutch 56. In the embodiment illustrated in FIGS. 2-4, for example,input member 64 includes a single non-magnetic feature 70. In theembodiment shown in FIG. 6 by comparison, output member 66 includes aplurality of non-magnetic features 70 b located radially outwardly of apoint equidistantly between non-magnetic features 70 a in input member64.

Input member 64 and output member 66 are not in contact, and may definetherebetween a substantially uniform gap 72. Gap 72 should be wideenough to permit a thin layer of medium 68 (see, e.g., FIG. 5A), such asa magnetically reactive powder (e.g., iron powder), to reside betweeninput and output members 64, 66. As shown in FIGS. 5A and 5B, thenon-magnetic properties of features 70 aid in concentrating anddirecting lines of magnetic flux 74 across gap 72.

Variable-engagement clutch 56 also includes a source of magnetic flux76, which may include, for example, an electromagnet mounted on theoutside of support 62 between input member 64 and support 62. In theillustrated embodiment, source 76 includes a wire-wound coil 78surrounded by a generally toroidal shell 80. As is well known, anelectrical current applied to coil 78 can generate a magnetic field inthe vicinity of source 76, the intensity of which is proportional to thelevel of current provided. Alternatively, source 76 may comprise otherarrangements, including, for example, a permanent magnet supplemented bya counteracting electromagnet so that clutch 56 will default to beingengaged should the electromagnet fail.

It is well known that lines of magnetic flux 74 travel a pathsubstantially through structures with known magnetic properties. Asillustrated in FIG. 4, lines of magnetic flux 74 exit rigid shell 80into input member 64, whereby flux 74 saturates input member 64. Uponsaturation, lines of magnetic flux 74 follow a path of least resistanceand traverse gap 72 into output member 66. The narrowest width offeatures 70 is best designed to be greater than the width of gap 72,thus preventing flux 74 from traversing features 70 and short-circuitingmedium 68. Upon entry into output member 66, flux 74 saturates outputmember 66 and then re-traverses gap 72 into input member 64. In theembodiment shown in FIG. 6, this process is repeated to weave flux 74across gap 72 between features 70 a and 70 b until the number ofnon-magnetic features is exhausted.

As shown in FIG. 5B, magnetically reactive particles 68 p can changeformation in relation to the intensity of the magnetic field, forexample, by aligning with lines of magnetic flux 74 as flux 74 traversesgap 72. Magnetically reactive particles 68 p under the influence of amagnetic field can link or lock into chains 83, increasing the shearforce and creating a mechanical friction against the surfaces of inputand output members 64, 66 facing gap 72. The increased shear andfriction result in a transfer of torque between input member 64 andoutput member 66.

In an embodiment, differential assembly 22 may also include a controlsystem having a pressure sensor 82 for monitoring the hydraulic fluidpressure generated by hydraulic pump 54 and a controller 84, such as anmicroprocessor-based electronic control unit (ECU), adapted to vary theelectric current supplied to variable-engagement clutch 56 to controlthe degree of hydraulic clutch 44 engagement in response to thehydraulic fluid pressure generated by hydraulic pump 54, such as in aclosed-loop fashion. Controller 84 may include sufficient memory tostore logic rules, generally in the form of a computer program, forcontrolling operation of variable-engagement clutch 56 and may beadapted to receive one or more inputs from various vehicle sources, suchas a speed sensor, steering sensor, torque sensor or other vehiclecontroller, to determine when to activate clutch 56. It will beappreciated by those skilled in the art that the present invention isnot limited to any particular type or configuration of ECU or to anyspecific control logic. Additionally, controller 84 may be integratedinto differential assembly 22 and adapted to receive information from avehicle communication bus, or may be contained in one or more vehiclecontrollers, such as the main vehicle ECU.

When it is desired to operate hydraulic clutch 44 by engagingvariable-engagement clutch 56, an appropriate electrical signal can betransmitted to source of magnetic flux 76 to create a magnetic field,which as described above, can alter the properties of medium 68 to causea transfer of torque between input member 64 and output member 66. In anembodiment, variable-engagement clutch 56 exhibits a nearly linearrelationship between its output torque and the current applied to source76, up to the magnetic saturation point of clutch 56. Accordingly, theamount of torque transferred between input and output members 64, 66 maybe selectively controlled by varying the current applied to source 76,such that a partial engagement may be achieved when it is desirable, ora full engagement may be achieved when it is needed and acceptable. Forexample, when only minimal torque transfer between outputs 28 and 30 isdesired, variable-engagement clutch 56 may be partially engaged, whereaswhen full axle lock is desired, variable-engagement clutch 56 may befully engaged. Gradual engagement of variable-engagement clutch 56 alsoeliminates or reduces vehicle lurch caused by conventional limited-slipdifferentials having an output-locking clutch that engages in a virtualON/OFF manner.

The input current to operate variable-engagement clutch 56 may beapplied in two parts: (i) an engagement current required to fully engagethe variable-engagement clutch 56; and (ii) a steady state currentrepresenting a predetermined current required to maintainvariable-engagement clutch 56 fully engaged. An unlimited number ofstrategies for controlling engagement of variable-engagement clutch 56may be generated, for example, by varying at least one of: (i) the levelof engagement current; (ii) the rate of application of engagementcurrent; and (iii) the rate of reduction of engagement current. Thegreater the magnitude and application rate of engagement current, thefaster the engagement of variable-engagement clutch 56. As previouslydescribed, the engagement of variable-engagement clutch 56 is, at leastin part, a function of the strength of the magnetic field generated bythe source of magnetic flux 74, which in turn is related to the electriccurrent applied to coil 78. When relatively fast engagement ofvariable-engagement clutch 56 is desired, the engagement current may behigher than the steady state current to overcome the inertial effects ofthe rotating input and output members 64, 66 coming up to speed. When arelatively slow engagement of variable-engagement clutch 56 is desired,the engagement current may be slowly ramped toward the steady statecurrent.

The application of input current to source 76 may also be accomplishedby pulse width modulating (PWM) the electrical signal provided bycontroller 84. According to this method, an electrical signal having apredetermined current, for example the current corresponding to themagnetic saturation point of variable-engagement clutch 56, is pulsed ata predetermined frequency, which results in a lower overall mean inputcurrent being applied to source 78. For example, without limitation, anelectrical signal with a current value of 6 amps could be pulsed 50% ofthe time resulting in approximately one-half of the input powerassociated with 6 amps being applied to source 76. As will beappreciated, pulse width modulating the engagement current may reducethe maximum power input to source 76 resulting in a more efficientoperation of variable-engagement clutch 56.

Because of the potentially linear (or substantially linear) relationshipbetween the application of current and output torque ofvariable-engagement clutch 56, it is possible to apply an input currentto source 76 that permits the output member 66 to slip relative to inputmember 64 resulting in clutch 56 being only partially engaged. Whenpartially engaged, a lesser amount of torque is transferred from inputmember 64 to output member 66 than would be transferred ifvariable-engagement clutch 56 were fully engaged. Thus, the resultingspeed at which output member 66 drives pump, and accordingly the outputpressure of pump 54, may be varied according to the input currentprovided to variable-engagement clutch 56.

As will also be appreciated, variable-engagement clutch 56 may beengaged to operate hydraulic pump 54 when there is substantially nodifference in speed between outputs 28 and 30—a feature useful invehicle stability control applications. For example, over-steer is acondition where a vehicle is making too tight of a turn for a givenvehicle speed, which may result in the vehicle spinning out of control.During over-steer, the difference in speed between outputs 28, 30 isrelatively low and is generally not indicative of a loss of traction ina drive wheel. Engagement of variable-engagement clutch 56 allows thevehicle to lock rotation of outputs 28, 30, which effectively speeds upthe inner drive wheel to correct the over-steer condition.

Referring to FIG. 7, a cross-sectional view of a torque coupling 102according to an embodiment of the present invention is shown. In theillustrated embodiment, torque coupling 102 is substantially similar todifferential assembly 22 in both structure and operation with at leastone exception, namely, torque coupling 102 does not include adifferential component 24. Instead, an input 104 is operativelyconnected to at least one friction disk 106 of a multi-disk clutch pack108 and an output 110 is operatively connected to at least one frictiondisk 112. Operation of torque coupling 102 is substantially similar tooperation of differential assembly 22 in that a hydraulic pump 114 isdriven by a variable-engagement clutch 116 operatively connected toinput 104 such that input 104 selectively and variably drives hydraulicpump 114 during engagement of variable-engagement clutch 116 toselectively compress clutch pack 108 and transfer torque between input104 and output 110.

Referring to FIGS. 8A-8D, a cross-sectional view of a vehicledifferential assembly 200 is shown according to an embodiment of thepresent invention. The differential assembly 200 includes ahydraulically-assisted, electronically-controlled, limited-slipdifferential that is capable of providing variable torque distributionbetween a pair of outputs 202, 204 driven by input 26. The outputs 202,204 may also be referred to as half-shafts. Accordingly, the half-shafts202, 204, if desired, can range, in operation, from a default, full-slipmode to a full-lock mode. Differential 200 may be provided as astand-alone assembly or, instead, if desired, may be integrated withanother vehicle system, such as a vehicle's ABS or stability controlpackage, to provide enhanced vehicle dynamics.

According to an embodiment of the invention, the vehicle differentialassembly 200 may generally operate in a similar manner as described inconnection with FIGS. 1-7, which include the following elements: input26, hydraulic pump 54, variable-engagement clutch 56, sump 58, bearings60, cylindrical support 62, input member 64, output member 66,short-circuiting medium 68, non-magnetic feature(s) 70, gap 72, magneticflux 74, source of magnetic flux 76, wire-wound coil 78, and controller84. The variable engagement clutch 56, may be, for example, a magneticparticle clutch, which engages the hydraulic pump 54 to the input shaft26 or a gear, such as, for example, a spur gear 79, connected to theinput shaft 26. Torque transmitted by the magnetic particle clutch 56can be proportional to electrical current provided to the magneticparticle clutch 56.

The vehicle differential assembly 200 is also shown to include a valve,which is shown generally at 75. Illustrated valve 75 is in fluidcommunication with hydraulic pump 54, sump 58, andhydraulically-actuated clutches, which are shown generally at 44 a, 44 band are associated with half shafts 202, 204, respectively. The valve 75may also include an actuator 77 that may receive commands fromcontroller 84 to direct fluid pressure to either one or both of thehydraulically-actuated clutches 44 a, 44 b. When the variable engagementclutch 56 is not energized, essentially no fluid will be pumped to theone or more hydraulically-actuated clutches 44 a, 44 b. According to anembodiment, the valve 75 may include, for example, a spring-centeredvalve, such as, for instance, a servo valve. According to an embodiment,the actuator 77 may include, for example, a single- or dual-coilsolenoid.

The hydraulically-actuated clutches 44 a, 44 b, which are shownschematically for purpose of illustration, each respectively include amulti-disk clutch pack 46 a, 46 b and a clutch pack-compressing actuator48 a, 48 b (e.g., a piston) that is movable in response to applicationof hydraulic fluid pressure from the hydraulic pump 54. An embodiment ofthe illustrated hydraulically-actuated clutches 44 a, 44 b isgenerically shown in FIGS. 8A-8D and provided by way of reference. Itwill be appreciated that other fluid power assisted clutchconfigurations that selectively and variably couple rotation of the halfshafts 202, 204 relative the drive wheel 16 a, 16 b may be employed inalternate configurations without departing from the scope of the presentinvention. For example, hydraulically-actuated clutches 44 a, 44 b maycomprise any type of axially-actuated clutch, such as, for example andwithout limitation, a single disk clutch, a multi-disk clutch or a coneclutch. A square jaw clutch or a spiral jaw clutch may also be used;however, these clutches do not necessarily proportionally transmittorque.

In the illustrated embodiment, each hydraulically-actuated clutch 44 a,44 b includes at least one first friction disk 50 a, 50 b connected forrotation with an outer surface 206 a, 206 b of the half shafts 202, 204and at least one second friction disk 52 a, 52 b connected for rotationwith a differential housing 208 a, 208 b respectively. Each differentialhousing 208 a, 208 b is respectively encompassed by and rotatably drivenby gears 210 a, 210 b. The gears 210 a, 210 b are driven by rotation ofa lay-shaft 212 including a pinion gear 214. The pinion gear 214 of thelay-shaft 212 is rotatably-driven by the ring gear 38 of the gearassembly 32.

When the vehicle 10 is driven, output of the hydraulic pump 54 may beselectively directed by valve 75 to the hydraulically-actuated clutches44 a, 44 b according to commands received at the actuator 77 from thecontroller 84. For example, controller 84 may direct the actuator 77 tocause the valve 75 to direct output of the hydraulic pump 54 to (a) thefirst hydraulically-actuated clutch 44 a, (b) the secondhydraulically-actuated clutch 44 b, or (c) both hydraulically-actuatedclutches 44 a, 44 b. The controller may also de-energize the variableclutch 54 such that the pump 54 will not output fluid. For example, theabove-described output of the hydraulic pump 54 may be represented byvarious combinations of an “X/Y position” signal sent from thecontroller 84 to the actuator 77 to vary fluid pressure applied to theclutch pack-actuators 48 a, 48 b. For example, the variable “X” may berelated to the application of torque to the drive wheel 16 a arisingfrom movement of the first clutch pack-actuator 48 a, and, the variable“Y” may be related to the application of torque to the drive wheel 16 barising from movement of the second clutch pack-actuator 48 b. In thefollowing description, the “off” and “on” position signals may besubstituted for either variable “X” or “Y” to show no application offluid pressure, or, an application of fluid pressure to the clutchpack-actuators 48 a, 48 b. Although the following description only showsthe terms “off” and “on,” it will be appreciated that the fluid pressureapplied to the clutch pack-actuators 48 a, 48 b need not be all ornothing; rather, it may vary in a functional manner so as to selectivelycontrol a desired amount of fluid pressure supplied to thehydraulically-actuated clutches 44 a, 44 b.

In the embodiment illustrated in FIG. 8A (which may be referred to as a“default state”), controller 84 is not sending current to the variableclutch 56. As a result, fluid is not pumped and no direct torque isapplied to each drive wheel 16 a, 16 b. Accordingly, when the valve 75is in a default state, each hydraulically-actuated clutch 44 a, 44 b isin a substantially open state, permitting full (or substantially full)slip of each half-shaft 202, 204 during operation of the vehicle 10. Inthis mode, the differential may function as a standard opendifferential.

As illustrated in the embodiment of FIG. 8B, an “on/off position” signalmay be sent from the controller 84 to variable clutch 56 and actuator77. The variable clutch 56 is shown engaged and the pump 54 is pumpingin this position. The “on/off position” signal moves the valve 75 to orinto a state such that fluid output is directed from the hydraulic pump54 to first clutch pack-compressing actuator 48 a, while blocking fluidoutput from the hydraulic pump 54 to second clutch pack-compressingactuator 48 b. As a result, torque is applied to the drive wheel 16 awhile no torque is applied to the drive wheel 16 b. Accordingly, thefirst hydraulically-actuated clutch 44 a is moved to a closed state andmay at least partially (or even fully) lock rotation of half shaft 202with the drive wheel 16 a while allowing slip of the half shaft 204 andrespective drive wheel 16 b during operation of the vehicle 10.

As illustrated in the embodiment of FIG. 8C, an “off/on position” signalis sent from the controller 84 to variable clutch 56 and to the actuator77. The variable clutch 56 is shown engaged and the pump 54 is pumpingin this position. The “off/on position” signal moves the valve 75 to orinto a state such that fluid output is directed from the hydraulic pump54 to the second clutch pack-compressing actuator 48 b, while impedingor blocking fluid output from the hydraulic pump 54 to the first clutchpack-compressing actuator 48 a. As a result, torque may be applied tothe drive wheel 16 b while no torque may be applied to the drive wheel16 a. Accordingly, the first hydraulically-actuated clutch 44 b is movedto a closed state and may at least partially or fully lock rotation ofhalf shaft 204 with the drive wheel 16 b while allowing slip of the halfshaft 202 and respective drive wheel 16 a during operation of thevehicle 10.

As illustrated in the embodiment of FIG. 8D, an “on/on position” signalis sent from the controller 84 to variable clutch 56 and to the actuator77. The variable clutch 56 is shown engaged and the pump 54 is pumpingin this position. The “on/on position” signal moves the valve 75 to orinto a state that directs fluid output from the hydraulic pump 54 toboth clutch pack-compressing actuators 48 a, 48 b. As a result, eachhydraulically-actuated clutch 44 a, 44 b is in a closed state topartially or fully lock both half-shafts 202, 204 and respective drivewheels 16 a, 16 b during operation of the vehicle 10.

By providing such an arrangement of the valve 75 and actuator 77 with adifferential assembly 200, a vehicle 10 may have improved torquevectoring capabilities, thereby, among other things, improving thestability control of a vehicle 10. Referring to FIG. 9, an embodiment ofa method for controlling the vehicle 10 having torque vectoringcapabilities is generally shown (and is labeled 300). Torque vectoringgenerally relates to the control of the rotational speed of inside oroutside drive wheels. For example, vectoring torque to: (a) one or bothinside drive wheels, or (b) one or both outside drive wheels, cancorrect, respectively, an over-steer or under-steer condition of avehicle 10. Differences in rotational speed of drive wheels cantypically occur when a vehicle enters a turn (e.g. the vehicle generallydeviates from a forward movement to a lateral movement). Although theabove-described embodiment in FIGS. 8A-8D only illustrate one outsidedrive wheel 16 a and one inside drive wheel 16 b, it will be appreciatedthat control of two or more outside and inside drive wheels 16, 17 maybe accomplished with a transfer case 19, such as generally describedabove.

When considering the description of FIGS. 8A-8D in view of thedescription associated with FIG. 9, the drive wheel 16 a may be referredto as an “outside wheel” and the drive wheel 16 b may be referred to asan “inside wheel.” As seen at step S.301, the vehicle 10 may beinitially placed/maintained in a default drive mode when no fluidpressure is applied to either hydraulically-actuated clutch 44 a, 44 b.Then, at step S.302, rotational speed of the outside drive wheel 16 aand the inside drive wheel 16 b is sensed or detected by, for example,wheel speed sensors 17 a, 17 b (FIGS. 8A-8D), respectively. At stepS.303, the rotational speed of each drive wheel 16 a, 16 b is comparedto determine if the rotational speeds are substantially the same (orwithin a specified difference). If the rotational speeds of the drivewheels 16 a, 16 b are substantially the same at step S.303, step S.303is returned to step S.301.

However, if the rotational speeds of the drive wheels 16 a, 16 b aredetermined to not be substantially the same (or within a specifieddifference) at step S.303, step S.303 is advanced to step S.304 where itis determined if both drive wheels 16 a, 16 b are rotating. At stepS.304, if it is determined that both drive wheels 16 a, 16 b arerotating, but at different speeds, step S.304 is advanced to step S.305;conversely, if it is determined that both drive wheels 16 a, 16 b arenot rotating, step S.304 is advanced to step S.308.

A difference in wheel speed is commonly experienced when a vehicle isentering a turn. When in a turn, the inside wheels 16 b travel rotateslower (i.e., travel on a smaller arc or circle) than the outside wheels16 a. Accordingly, the speed of the wheels 16 a, 16 b provideinformation regarding the difference of circle diameter that the wheels16 a, 16 b are traveling on. When the diameters are compared with anangle of the steering wheel, slip angles for the front wheels and rearwheels are computed, for example, by the controller 84. If the slipangles are equal, the vehicle 10 is in a neutral steering state and nocorrection is required. However, if, the slip angle of the front wheelsexceeds that of the rear wheels, the vehicle 10 is under-steering andmore torque may need to be sent to the outside wheels 16 a. Conversely,if the slip angle of the rear wheels exceeds that of the front wheels,the vehicle 10 is over-steering and more torque may need to be sent tothe inside wheels 16 b.

Accordingly, at step S.305, it is determined if the vehicle 10 is in anunder-steer or over-steer situation. If the vehicle 10 is in anunder-steer situation, step S.305 is advanced to step S.306 where thevalve 75 directs fluid pressure to the hydraulically-actuated clutch 44a to increase torque to the outside drive wheel 16 a. Step S.306 is thenadvanced to step S.302 to reassess the rotational speed of the wheels inthe form of a feedback control loop and the cycle can be generallyrepeated.

If, at step S.305, it is determined that the vehicle 10 is in anover-steer situation, step S.305 is advanced to step S.307. At stepS.307, the valve 75 directs fluid pressure to the hydraulically-actuateclutch 44 b to increase torque to the inside drive wheel 16 b. StepS.307 is then advanced to step S.302 to reassess the rotational speed ofthe wheels in the form of a feedback control loop and the cycle can begenerally repeated.

If a method, such as discussed in connection with FIG. 9, is advanced toa step such as S.308, the controller 84 may, for example, havedetermined (e.g., from one of the wheel speed sensors 17 a, 17 b) thatone of the drive wheels 16 a, 16 b is not rotating. When such acondition occurs, the controller 84 can be configured to instruct thevalve 75 (e.g., at step S.308) to direct fluid pressure to bothhydraulically-actuated clutches 44 a, 44 b to lock (or substantiallylock) rotation of the half-shafts 202, 204 so that rotation of bothdrive wheels 16 a, 16 b become substantially the same. Step S.308 canthen be advanced to step S.302 to generally repeat the cycle as notedabove.

The method, as generally shown at 300, may be a program that is storedin the controller 84. Accordingly, as described above, the wheel speedsensors 17 a, 17 b may provide yet another input to the controller 84that works in cooperation with the hydraulic pump 54,variable-engagement clutch 56, pressure sensor 82, and valve 75 tocontrol torque vectoring to the drive wheels 16 a, 16 b. Superiorstability of the vehicle 10 is thereby obtained by applying the method300.

The present invention has been particularly shown and described withreference to the foregoing embodiments, which are merely illustrative ofthe best modes for carrying out the invention. It should be understoodby those skilled in the art that various alternatives to the embodimentsof the invention described herein may be employed in practicing theinvention without departing from the spirit and scope of the inventionas defined in the following claims. It is intended that the followingclaims define the scope of the invention and that the method andapparatus within the scope of these claims and their equivalents becovered thereby. This description of the invention should be understoodto include all novel and non-obvious combinations of elements describedherein, and claims may be presented in this or a later application toany novel and non-obvious combination of these elements. Moreover, theforegoing embodiments are illustrative, and no single feature or elementis essential to all possible combinations that may be claimed in this ora later application.

1. A method of controlling stability of a vehicle comprising the stepsof: determining rotational speed of at least inside and outside drivewheels that are rotatably-controlled by respectivehydraulically-actuated clutches; and if the rotational speed of theinside and outside drive wheel is not substantially the same, engaging avariable engagement clutch operatively connected to a hydraulic pump,wherein a degree of clutch engagement of the variable engagement clutchis based on a degree of desired clutch engagement for at least one ofthe hydraulically-actuated clutches; selectively driving the hydraulicpump by engagement of the variable engagement clutch to generatehydraulic fluid pressure; and applying fluid pressure from the hydraulicpump that is selectively and variably directed by a valve to thehydraulically-actuated clutches for adjusting or vectoring torque of oneof or both the inside and outside drive wheels.
 2. The method accordingto claim 1, further comprising the step of if the rotational speed ofthe inside and outside drive wheels is substantially the same, applyingno fluid pressure to the hydraulically-actuated clutches to maintain thehydraulically-actuated clutches in an open state that allows full slipof outputs that are respectively connected to the inside and outsidewheels.
 3. The method according to claim 1, wherein the applying stepincludes selectively or variably applying fluid pressure to one of thehydraulically-actuated clutches that is operatively connected to theoutside drive wheel when the vehicle is in an under-steer situation. 4.The method according to claim 3, including: determining the speed of theoutside drive wheel; and adjusting the application of fluid pressure toone of the hydraulically actuated clutches that is operatively connectedto the outside drive wheel based at least in part on the speed of theoutside drive wheel.
 5. The method according to claim 1, wherein theapplying step includes selectively or variably applying fluid pressureto one of the hydraulically-actuated clutches that is operativelyconnected to the inside drive wheel when the vehicle is in an over-steersituation.
 6. The method according to claim 5, including: determiningthe speed of the inside drive wheel; and adjusting the application offluid pressure to one of the hydraulically actuated clutches that isoperatively connected to the inside drive wheel based at least in parton the speed of the inside drive wheel.
 7. The method according to claim1, wherein the applying step includes selectively or variably applyingfluid pressure to both hydraulically-actuated clutches to lock rotationof the inside and outside drive wheels with the rotational movement ofhalf-shafts that are driven by an input.
 8. The method according toclaim 1, wherein the degree of clutch engagement of the variableengagement clutch is controlled by varying at least one of: (a) a levelof current provided to the variable engagement clutch; (b) a rate ofapplication of current provided to the variable engagement clutch; and(c) a rate of reduction of current provided to the variable engagementclutch.
 9. The method according to claim 1, wherein engaging thevariable engagement clutch includes pulsing a current at a predeterminedfrequency to the variable engagement clutch.
 10. The method according toclaim 1, wherein the variable engagement clutch includes a magneticparticle clutch.
 11. The method according to claim 1, wherein thevariable engagement clutch includes a rotatable input member, arotatable output member operatively connected to the hydraulic pump, anda medium disposed between the input and output members.
 12. The methodaccording to claim 11, wherein the medium is a magneto-restrictivefluid.
 13. The method according to claim 11, wherein the medium is anelectro-restrictive fluid.
 14. The method according to claim 11,including creating a magnetic field to cause a transfer of torquebetween the input member and output member.
 15. The method according toclaim 1, including: moving an actuator that is moveable in response tothe application of fluid pressure from the hydraulic pump to thehydraulically-actuated clutches; and compressing a multi-disk clutchpack of the hydraulically-actuated clutches.
 16. The method according toclaim 1, wherein a gear assembly is connected to the inside and outsidedrive wheels, the gear assembly including a pair of side gears and aring gear having a pair of pinion gears that mesh with the side gears, afirst side gear connected to the inside drive wheel, and a second sidegear connected to the outside drive wheel.
 17. The method according toclaim 1, wherein the valve includes an actuator configured to receivecommands from a controller and to direct fluid pressure to thehydraulically actuated clutches.
 18. The method according to claim 17,wherein the actuator comprises a single or dual coil solenoid.
 19. Themethod according to claim 1, wherein the valve comprises aspring-centered valve.